The invention concerns an apparatus for generating and/or detecting radio frequency (RF) B1 fields in an investigational volume of a magnetic resonance (MR) device, wherein the apparatus comprises a surface coil.
Current sheet antennas are used for hyperthermia treatments as disclosed in publications by Johnson (U.S. Pat. No. 4,589,422, GB 2 135 891 and U.S. Pat. No. 4,600,018).
Nuclear magnetic resonance (NMR) plays an important role in analytical chemistry as well as in medical imaging methods.
In NMR methods, a sample to be investigated is located in a static B0 field and is exposed to one or more radio frequency (RF) pulses. The RF signals emanating from the sample are subsequently detected and evaluated.
Volume coils, such as the so-called TEM resonator (see WO 00/72033 A2), are conventionally used to generate (Tx) and detect (Rx) the RF fields and signals. The TEM resonator has a radially closed RF shield which comprises a plurality of individual interior resonance elements which are capacitively connected to the RF shield. Magnetic coupling of the individual resonance elements produces a transverse resonant mode.
DE 101 24 465 A1 discloses a volume coil with decoupled resonator elements, wherein all resonator elements are each individually fed with a defined phase and amplitude. Such volume coils are known as birdcage coils and have conductor surfaces disposed along the B0 field. Feeding of the individual resonance elements via power splitters and phase shifters is also known (EP 1 273 926 A2 or U.S. Pat. No. 6,411,090 B1) to thereby obtain control over the B1 field distribution within a particular volume.
In open MRI (magnetic resonance imaging) systems, so-called planar RF antennas (planar surface coils, planar coils) are used (see DE 101 24 737 A1). They consist of a rectangular, disc-shaped, or annular conductor element whose surface normal (or annular axis) is orthogonal: to the B0 field. The planar RF antennas are capacitively connected to a ground shield. The planar RF antenna generates a B1 field which is orthogonal to its antenna plane and therefore transverse to B0. Suitable switching of the capacitances generates a linearly or circularly polarized RF-B1-field.
In addition to volume coils, local coils are also conventionally used (see ISMRM abstr. 2002, no. 321/322). Local coils generate and detect NMR signals within a limited investigational volume of the MRI device. The local coils are formed from so-called planar strip arrays. They consist of a plurality of individual flat, rectangular conductors which are capacitively connected to ground at regular intervals. The ground reference is realized with a flat, metallic conducting surface. The individual conductors are separately fed and may be decoupled from each other using impedance networks (inductances and capacitances).
To heat up individual human body parts (hyperthermy), current sheet antennas (CSA) are conventionally disposed with their outer side on the surface of the body part and fed with an alternating current. In particular, rectangular tubular applicators are known which operate at a frequency range of 27 to 450 MHz. The electric RF fields which are generated by these antennas are used to heat up the tissue to approximately 40° C. or more (compare Johnson loc. cit.). The applicators have a defined resonance frequency and are optimised to effectively radiate the electric RF field. Magnetic RF fields are not used. The applicators can be connected to arrays and be operated with predetermined amplitude and phase using separate transmitting channels to influence the field distribution and depth effect.
It is the underlying purpose of the present invention to provide an NMR transmitting and receiving apparatus with improved RF properties, in particular, one in which the transverse RF-B1-field can be distributed in a defined, controlled manner.